demux/subtitles: capture language attribute from SSA/ASS files

... for language identification.

this info property has been supported by libass since v0.10.0. it is currently
a 2-char iso-639-1 code.

libass commit adding support:
libass/libass@c979365

discussion about enhancing the attribute to support 3-char iso-639-2 codes,
possibly bcp-47: libass/libass#404

modules/demux/subtitle.c

@@ -1239,11 +1239,19 @@ static int  ParseSSA( vlc_object_t *p_obj, subs_properties_t *p_props,
          * Dialogue: Layer#,0:02:40.65,0:02:41.79,Wolf main,Cher,0000,0000,0000,,Et les enregistrements de ses ondes delta ?
          */
 
-        /* The output text is - at least, not removing numbers - 18 chars shorter than the input text. */
+        /* The output text is always shorter than the input text. */
         psz_text = malloc( strlen(s) );
         if( !psz_text )
             return VLC_ENOMEM;
 
+        /* Try to capture the language property */
+        if( sscanf( s, "Language: %[^\r\n]", psz_text ) == 1 )
+        {
+            free( p_props->psz_lang ); /* just in case of multiple instances */
+            p_props->psz_lang = psz_text;
+            psz_text = NULL;
+        }
+        else
         if( sscanf( s,
                     "Dialogue: %15[^,],%d:%d:%d.%d,%d:%d:%d.%d,%[^\r\n]",
                     temp,